I've been using BillMonk a lot recently. It's reached the verb stage very quickly, and I think they've done a great job solving a common need.
Nonetheless, two things annoy me about the site, and I thought I'd post them publicly:
Right now they have two options for how much each person paid: an Even share or an Exact amount. Even is very useful, especially because they let you specify multiple shares for one person (solving the "boyfriend problem").
But I think they didn't fully think through Exact- what I find myself usually wanting to do is to specify an extra amount for one person rather than giving their total. I think the most common use case for an uneven bill is that somebody got a dessert or drink, but shared equally in the rest of the bill.
With Exact in that case, you're basically asking a human to subtract the extra item, divide by the total number of people, and add the extra back to get the correct Exact amount for the person in question.
That should clearly be done by the website (javascript-only would be fine).
They have email and SMS options, but in both the Shared Bill mechanism (which, as I mentioned, I use most often) is unwieldy and requires all the participants to SMS or email.
The Sidekick's web browser is improving every day, and has some javascript, but BillMonk's Shared Bill form is still past its abilities.
When it comes down to it, it's bad form (ha) to have any form (ha) that requires javascript to function, and particularly an app like this where mobile use is so clearly of utility.
On the other hand, maybe they make it inaccessible to encourage the use of the excellent Group/Victim functionality, which we employed (via spreadsheet) for lunches at Microsoft. But all forms should work in lynx anyway.